The main goal of this investigation was to extend the prior work which implicated AP-2α as a possible tumor suppressor. Inasmuch as in vivo gene delivery of AP-2α inhibited intestinal polyps in the Apcmin mouse, and protected against the development of anemia and splenomegaly, we can conclude that our findings are consistent with a tumor suppressor role for this transcription factor. With appropriate modification of the in vivo gene delivery protocol, such as shortening the duration between injections, it might be feasible to inhibit polyp formation to a greater extent than observed here, although this possibility awaits experimental verification.
Consistent with its proposed tumor suppressor function, AP-2αexpression tended to be lower in polyps than adjacent normal-looking tissue, although this was not universally true. By the time polyps arise in Apcmin
mice the adjacent normal-looking tissue is scarcely “normal”, and microadenomas or other precursor lesions might conceivably affect AP-2α levels. Moreover, immunoblotting and immunohistochemical studies might not provide the complete picture. Inhumancolorectal cancer cells transfected with exogenous AP-2α, immunoblotting of nuclear extracts revealed high levels of AP-2α without changes in β-catenin or TCF-4; however, co-immunoprecipitation experiments revealed that β-catenin/TCF-4 nuclear interactions were attenuated, whereas AP-2αβ-catenin/APC interactions had increased [11
]. We extended these observations and showed, for the first time in vivo
, that AP-2α/APC/β-catenin interactions were detected in the intestinal mucosa of Apcmin
mice and in a primary human colorectal cancer.
We postulate that early stages of colorectal cancer might benefit most from therapy targeted at AP-2α, because the AP-2α/APC/β-catenin complex requires at least one full-length copy of APC (), and in late-stage colon cancers loss of heterozygosity can delete both wild type APC alleles [23
]. APC is known to shuttle in and out of the nucleus [24
]; thus, it is possible that nuclear APC, along with AP-2α, sequesters β-catenin away from TCF transcription factors as a means of controlling β-catenin/TCF signaling. Consistent with this paradigm, polyps with low levels of AP-2α expression had high levels of Cyclin D1, a down-stream β-catenin/TCF target. However, it is premature to conclude from the present investigation that AP-2α/APC/β-catenin complex formation was the only (or even primary) mechanism for tumor suppression. For example, Cyclin D1 and other downstream β-catenin/TCF targets might be influenced by AP-2α as a transcriptional regulator. Additional studies are warranted to clarify the precise mechanisms involved. The current preclinical model and in vivo
gene delivery approach has advantages in terms of simplicity and speed, but it is not ideal in that AP-2α was targeted primarily to liver. A conditional model targeting AP-2α to the intestinal and/or colonic mucosa might provide more definitive insights into the role of AP-2α as a tumor suppressor in the gastrointestinal tract.
In summary, we have shown for the first time that AP-2α suppresses the development of intestinal tumors arising spontaneously in a mouse model of dysregulated Wnt signaling. The findings are consistent with studies in primary human colon cancer and cultured colorectal cancer cells indicating that AP-2α associates with APC/β-catenin, with the potential to disrupt β-catenin/TCF interactions. Further work is needed to corroborate whether this mechanism might provide a therapeutic avenue in clinical cases of colorectal cancer.